A New and Versatile Synthesis of 1,3-Dioxan-5-yl-pyrimidine and Purine Nucleoside Analogues

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A New and Versatile Synthesis of 1,3-Dioxan-5-yl-pyrimidine and

Purine Nucleoside Analogues

Claudia Sorbia Adolfo Prandia Umberto M. Battistia Silvia Franchinia Andrea Corniab Jan Balzarinic

Lak Shin Jeongd

Sang Kook Leed

Jayoung Songd

Livio Brasili*a

a_{Department of Life Sciences, University of Modena & Reggio Emilia, Via G. Campi 183, 41125 Modena, Italy}

[email protected]

b_{Department of Chemical and Geological Sciences, University of Modena & Reggio Emilia, Via G. Campi 183, 41125 Modena, Italy} c_{Rega Institute for Medical Research, KU Leuven, Minderbroedersstraat 10, 3000 Leuven, Belgium}

d_{College of Pharmacy, Seoul National University, Seoul 151-742, Republic of Korea}

O ONa + Br OEt OEt O O HO base O O O O OTs

- all 1,3-dioxane based nucleoside analogues were obtained

- both trans and cis diastereoisomers were isolated and structurally characterized

Received: 14.11.2014

Accepted after revision: 22.12.2014 Published online: 22.01.2015

DOI: 10.1055/s-0034-1380112; Art ID: st-2014-d0944-l

Abstract 1,3-Dioxan-5-yl pyrimidine and purine nucleoside analogues were prepared following a new and versatile synthetic strategy. These analogues were synthesized via nucleophilic addition of the selected nucleobase to a 1,3-dioxane scaffold that presents an appropriate leav-ing group in position 5. In particular cis and trans isomers of purine/py-rimidine nucleosides and their halogenated homologues were ob-tained. NMR experiments, carried out on the cis isomers, led to assignment of an equatorial orientation to the 2-hydroxymethyl group and axial orientation to the nucleobase in position 5 of the 1,3-dioxane. The trans isomers showed a diequatorial orientation of these groups. These assignments were confirmed by X-ray crystallographic studies.

Key words nucleosides, 1,3-dioxane, purines, pyrimidines, antiviral

The synthesis and biological evaluation of nucleoside

analogues have been a very active research area for a

num-ber of years.

_{Among them, a well-known class of}

mole-cules endowed with antiviral or anticancer activity was

ob-tained by replacing the 2-deoxyribose moiety with a

1,3-di-oxolane

_{or 1,3-oxathiolane ring.}

_{Based on the results}

reported, we focused part of our research on the synthesis

of nucleoside analogues having these novel sugar-replacing

rings. Furthermore, with the aim to investigate the

proper-ties of higher homologues of the 1,3-dioxolane-based

nu-cleosides, 1,3-dioxane analogues were also considered

(Fig-ure 1).

Although 1,3-dioxan-5-yl pyrimidines were previously

reported as potential anti-HIV nucleoside alike

com-pounds,

_{we felt that the true value of a structural}

modifi-cation concerning the sugar portion would only be

com-pletely revealed when all nucleoside analogues were

pro-duced and evaluated. This consideration is supported by the

case of 1,3-oxathiolane-based nucleosides in which only

the cytosine analogue showed a good antiviral (i.e., HIV,

HBV) potency and selectivity.

_{Supported by this evidence,}

we developed a novel synthetic approach to obtain the

cor-responding purine derivatives. Unlike the previously

re-ported methods, the synthetic strategy presented herein

al-lowed us to isolate and fully characterize both cis and trans

isomers of all targeted derivatives. The 1,3-dioxane-based

nucleosides were obtained by reacting each purine and

py-rimidine base with a key intermediate in the last step of the

synthetic pathway. This strategy avoids the expensive and

time-consuming Mitsunobu-type condensation of

bis-1,3-trityloxy-2-propanol with each nucleobase in the first step

of the synthetic path.

_{In addition, it overcomes the}

de-manding separation of the cis/trans diastereoisomers to be

performed on the precious and hard-to-handle nucleoside

mixture. Moreover, the Mitsunobu reaction cannot be

ap-plied in the case of purines due to their poor solubility in

the suitable reaction solvents. At present,

1,3-dioxan-5-yl-purine nucleosides have never been isolated and

character-ized. In this work we developed a different synthetic

ap-proach in order to obtain a large variety of cis- and

trans-pyrimidine/purine nucleoside analogues by using a

com-Figure 1 O O base 1,3-oxathiolane- based nucleosides S O base _O O base 1,3-dioxolane-based nucleosides 1,3-dioxane-based nucleosides HO HO HO

SYNLETT0936-52141437-2096

© Georg Thieme Verlag Stuttgart · New York

2015, 26, A–F

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mon intermediate (Scheme 1). Diethyl acetal 1,

_required

as starting material, was prepared from sodium benzoate

and bromoacetaldehyde diethyl acetal. Synthesis of the

1,3-dioxane ring was accomplished by Lewis acid mediated

condensation of 1 with glycerol to give compound 2.

_The

reaction proceeds without stereoselectivity and

regioselec-tivity preferences, giving cis (2a)

_{and trans (2b)}

_isomers

in comparable yields as well as their corresponding

1,3-di-oxolane isomers. The molar ratio of

1,3-dioxane/1,3-dioxol-ane cis,trans-diastereoisomeric mixture was 55:45.

Howev-er, due to the marked difference in polarity, the single

dias-tereoisomers of 2 were easily separated by flash column

chromatography and obtained in high purity free from their

lower homologues (1,3-dioxolanes). Treatment of 2a and 2b

with p-toluenesulfonyl chloride gave the key intermediates

3a and 3b,

_{respectively. Nucleophilic displacement of the}

tosyl group by the selected nucleobase produced an

inver-sion of the configuration at C-5 of the 1,3-dioxane and

fur-nished trans (4a–13a) and cis isomers (4b–13b),

re-spectively, of the desired nucleosides. Deprotection of the

hydroxyl group in position 2 of the 1,3-dioxane with NH

–

H

O and the subsequent crystallization gave the final

pyrimidine nucleosides as pure trans (14a–21a) or cis

dia-stereoisomers (14b–21b; Scheme 1,Table 1).

The desired adenine analogues 22a and 22b

_were

ob-tained by treatment of the parent compounds 12a and 12b

with saturated NH

–H

O, in a reactor at 100 °C. Compounds

13a and 13b were transformed into the final guanine

nu-cleosides 23a and 23b

_{by heating at 100 °C in the}

pres-ence of aqueous NaOH–methanol solution (Scheme 1).

Stereochemical and conformational assignments were

based on NMR studies. The relative chemical shifts of the

H-5′ (1,3-dioxane) and H-6 (pyrimidine) or H-8 (purine) were

Table 1 Synthesized Compounds28

Nucleoside Nucleobase Nucleobase abbreviation

14a uracil U

14b uracil U

15a 5-chlorouracil 5ClU

15b 5-chlorouracil 5ClU

16a 5-fluorouracil 5FU

16b 5-fluorouracil 5FU

17a 5-bromouracil 5BrU

17b 5-bromouracil 5BrU

18a 5-iodouracil 5IU

18b 5-iodouracil 5IU 19a thymine T 19b thymine T 20a cytosine C 20b cytosine C 21a 5-fluorocytosine 5FC 21b 5-fluorocytosine 5FC 22a adenine A 22b adenine A 23a guanine G 23b guanine G

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taken into account to assign the cis/trans configurations to

all final compounds 14–23 (Figure 2 and Table 2). In

partic-ular, the H-5′ signal of the trans isomers appears downfield

with respect to that of the corresponding cis isomers. This

result is probably due to the deshielding effect of the

1,3-dioxane oxygen atoms, suggesting an axial orientation of

H-5′. Alternatively, in the cis isomers the H-5′ proton is

ar-ranged equatorially so that the nucleobase presents an axial

orientation. The deshielding effect of the oxygen atoms is

exerted also on H-6 of the pyrimidine or on H-8 of the

pu-rine. In particular, when the base is axially oriented, as in

the cis isomers, the signal of these protons falls at lower

field with respect to that of the corresponding trans isomer.

Moreover, the pattern of all H-5′ protons unambiguously

confirmed that in all cis isomers the nucleobase is axially

oriented while in trans isomers the base has equatorial

ori-entation (Figure 2). In fact, for the cis isomers, the H-5′ peak

is a broadened singlet with two small coupling constants.

This evidence is in good agreement with an equatorial

ori-entation of this proton when 2′,5′-disubstituted

1,3-diox-anes show a C-2′,C-5′ cis configuration. By contrast, the H-5′

signal of the trans isomers appears as a defined multiplet,

with two large coupling constants that indicate its axial

ori-Scheme 1 Reagents and conditions: i) 18-crown-6, DMF, 160 °C, 6 h; (ii)

CoCl₂, TMSCl, glycerol, MeCN, r.t., 0.1 h; iii) flash column chromatogra-phy (cyclohexane–EtOAc, 70:30); iv) TsCl, Et3N, CH2Cl2, 0 °C to r.t., 12 h; v) K2CO3, selected pyrimidine or purine, 18-crown-6, DMF, 160 °C, 24 h; vi) NH3–H2O, r.t., 5 h (4a–11a and 4b–11b) or NH3–H2O, 100 °C, 12 h (12a,b) or 40% NaOH–MeOH, 100 °C, 5 h (13a,b).

O ONa + Br OEt OEt i) O OEt OEt O ii) O O O O iii) O O O O OH + O O O O OH OH O O O O OTs O O O O OTs O O O O base 1 2 2a O O O O OH 2b 3a 3b O O O O base 4a–13a 4b–13b iv) iv) v) v) HO O O base 14a–23a vi) vi) HO O O base 14b–23b 2c N N N N R N N N N R NH2 NH N O O NH N O O N N NH2 O R base: R 4, 14 R = H 5, 15 R = Cl 6, 16 R = F 7, 17 R = Br 8, 18 R = I 10, 20 R = H 11, 21 R = F 9, 19 12 R = Cl 22 R = NH2 13 R = Cl 23 R = OH 55:45

Table 2 1_{H NMR Chemical Shifts (multiplicity) of H-5′, H-6, or H-8 of} the 1,3-Dioxane-Based Nucleoside Analogues 14–23a

Compound Base Isomer H-5′ H-6b _{or H-8}c

14a U trans 4.32–4.51 (m) 7.72 (d)

14b U cis 4.29 (br s) 8.15 (d)

15a 5ClU trans 4.41–4.52 (m) 8.16 (s)

15b 5ClU cis 4.31 (br s) 8.47 (s)

16a 5FU trans 4.39–4.49(m) 8.11 (d)

16b 5FU cis 4.30 (br s) 8.37 (d)

17a 5BrU trans 4.39–4.49 (m) 8.20 (s)

17b 5BrU cis 4.32 (br s) 8.45 (s)

18a 5IU trans 4.42–4.58 (m) 8.22 (s)

18b 5IU cis 4.33 (br s) 8.61 (s) 19a T trans 4.41–4.58 (m) 7.62 (s) 19b T cis 4.31 (br s) 8.04 (s) 20a C trans 4.43–4.59 (m) 7.61 (d) 20b C cis 4.30 (br s) 8.11 (d) 21a 5FC trans 4.45–4.61 (m) 8.03 (d) 21b 5FC cis 4.30 (br s) 8.28 (d) 22a A trans 4.61–4.82 (m) 8.21 (s) 22b A cis 4.51 (br s) 8.40 (s) 23a G trans 4.31–4.50 (m) 7.63 (s) 23b G cis 4.29 (br s) 7.88 (s) a 1_{H NMR: 400 MHz, DMSO-d}

6 as solvent, TMS as internal reference. b _{Pyrimidine derivatives.}

c _{Purine derivatives.}

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entation. These results were confirmed by

_{H NMR studies}

on 2′,5′-disubstituted 1,3-dioxane analogues.

_{The NMR}

structural elucidations were further supported by X-ray

crystallographic studies. The crystals of 22b, obtained from

aqueous solution, were analyzed by single-crystal X-ray

dif-fraction at room temperature. The crystals were monoclinic

(space group P2

/c) with four molecules per unit cell. Bond

lengths and angles are unexceptional, as compared with

those found in cyclohexyl

_{and pyranosyl}

_{derivatives of}

adenine. The 1,3-dioxane ring adopts a chair conformation,

with the adenine and hydroxymethyl groups in axial and

equatorial positions, respectively (Figure 3).

Figure 3 Partially labeled ORTEP-3 plot of 22b, with displacement

el-lipsoids at 40% probability level and H atoms drawn as spheres with ar-bitrary radius.14 _{Only H atoms bound to heteroatoms are labeled.}

The purine base is in the anti orientation with respect

to the 1,3-dioxane ring, as shown by the torsion angle C7–

C6–N1–C5, –151.31(7)°. The molecular structure is

remark-ably similar to that of

trans-9-(2-ethoxy-1,3-dioxan-5-yl)adenine, which, however, features the ethoxy substituent

in axial position.

In the crystal lattice, complementary donor–acceptor

interactions link molecules in chains that run parallel to the

c axis. The hydroxyl oxygen atom O3 is hydrogen-bonded to

the imidazole nitrogen atom N2 of a neighboring molecule

[O3–HO3···N2 2.7759(11) Å]. In turn, the exocyclic amino

group of the latter acts as a hydrogen donor towards one of

the 1,3-dioxane ring oxygens [N5–HN5a···O2 3.0265(11) Å].

These chains are connected to each other via a third

hydro-gen bond that involves the remaining amino hydrohydro-gen and

the pyrimidine nitrogen atom N3 [N5–HN5b···N3

3.0645(10) Å]. The crystal structure is further stabilized by

short C–H···O contacts and base-stacking interactions.

All synthesized nucleosides 14–23 were evaluated for

their potential activity against a variety of viruses following

the previously described procedures.

_{None of the}

com-pounds showed significant antiviral activity against human

immunodeficiency virus type 1 and 2 (HIV-1, HIV-2),

her-pes simplex virus type 1 and 2 (HSV-1, HSV-2), vaccinia

vi-rus and vescicular stomatitis vivi-rus (VSV) in infected MT-4

(for HIV) or HEL (other viruses) cell cultures. Moreover no

microscopically visible cytotoxicity was observed at the

highest concentrations tested (i.e., 200 μM) of all

com-pounds. The compounds were also evaluated for cytotoxic

effects towards several human cancer cell lines such as

HCT116 (colon), A549 (lung), SNU638 (stomach), PC3

(pros-tate), SK-Hep-1 (liver), using sulforhodamine B (SRB)

pro-tein staining method.

_{None of the tested compounds}

showed significant antitumor activity, with an EC

value

> 100 μM.

In summary, a new and versatile synthesis of achiral

1,3-dioxan-5-yl-based nucleosides

(pyrimidin-1-yl/purin-9-yl) has been developed. This synthetic strategy allows

both cis and trans diastereoisomers of all purine and

pyrim-idine analogues to be obtained. Furthermore,

1,3-dioxan-5-yl purines were isolated and structurally characterized for

the first time. None of the compounds possesses significant

antiviral and antitumor activity in the biological assays

performed. Further or different modifications have to be

in-troduced to unlock the antiviral/antitumor activity.

Howev-er, this synthetic approach may represent a valuable tool for

obtaining new analogues of this class of compounds.

Acknowledgment

The antiviral and cytostatic evaluations were performed by Leentje Persoons, Frieda De Meyer, Kristien Erven, Kris Uyttersprot, and Liz-ette van Berckelaer and supported by a grant of the KU Leuven (GOA 10/14).

Supporting Information

Supporting information for this article is available online at http://dx.doi.org/10.1055/s-0034-1380112. Supporting InformationSupporting Information

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(17) Benzoyloxyacetaldehyde Diethyl Acetal (1)

Potassium benzoate (125.2 mmol, 20.0 g) was added to a solu-tion of bromoacetaldehyde diethyl acetal (160.0 mmol, 24.4 mL) and 18-crown-6 ether (catalytic amount) in anhydrous DMF (25 mL), and the mixture was refluxed for 6 h. Then, after cooling to r.t., H2O was added, and the mixture was extracted three times with EtOAc. The combined extracts were washed with H2O, dried (Na2SO4), and concentrated under vacuum. The residue was dried azeotropically with toluene to give benzoyloxyacetal-dehyde diethyl acetal (24.72 g, 104.0 mmol, 83%). This product was used in the next step without further purification. Dark oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 1.22 (t, J = 7.2 Hz, 6 H, 2 × CH3), 3.54–3.68 (m, 2 H, CH2CH3), 3.68–3.80 (m, 2 H, CH2CH3), 4.35 (d, J = 5.4 Hz, 2 H, CH2OCO), 4.84 (t, J = 5.4 Hz, 1 H, CH), 7.43 (dd, J = 7.6, 7.7 Hz, 2 H, CH-3, CH-5 Ph), 7.55 (t, J = 7.6 Hz, 1 H, CH-4 Ph), 8.05 (d, J = 7.7 Hz, 2 H, CH-2, CH-6 Ph). 13_{C NMR (100} MHz, CDCl3): δ = 15.0 (2 CH3), 62.2 (2 CH2), 64.1 (CH2OCO), 99.4 (CH), 128.1 (C-3, C-5 Ph), 129.4 (C-2, C-6 Ph), 129.7 (C-1 Ph), 132.8 (C-4 Ph), 166.0 (CO). HRMS-APCI: m/z calcd for C13H19O4+ [M + H]+_{: 239.1278; found: 239.1280.}

(18) (5-Hydroxy-1,3-dioxan-2-yl)methyl Benzoate (2)

To a solution of CoCl2 (9.7 g, 75.0 mmol) in anhydrous MeCN (100 mL), benzoyloxyacetaldehyde diethyl acetal (1, 33.3 g, 140.0 mmol), TMSCl (19.0 mL, 149.0 mmol), and glycerol (19.3 mL, 265.0 mmol) were added at r.t. under stirring. After 12 h the reaction was stopped, the mixture was extracted three times with EtOAc, and the extracts were collected and washed

with NaHCO3 (5%). The organic layer was dried (Na2SO4), fil-tered, and the solvent was evaporated under vacuum to give an oily residue. Purification and separation of two diastereoiso-mers 2a and 2b was achieved by flash column chromatography (cyclohexane–ethyl acetate, 70:30): 5.00 g of cis isomer 2a (21.0 mmol, 15%), 5.34 g of trans isomer 2b (22.4 mmol, 16%), and 10.0 g of [4-(hydroxymethyl)-1,3-dioxolan-2-yl]methyl benzo-ate (2c, 42.0 mmol, 30%) as an inseparable cis/trans diastereo-isomeric mixture (50:50) were obtained. Molar ratio 1,3-diox-anes/1,3-dioxolanes = 55:45.

(19) cis-(5-Hydroxy-1,3-dioxan-2-yl)methyl Benzoate (2a) Yellow oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 3.10 (br s, 1 H, OH), 3.52–3.61 (m, 1 H, CH-5 diox), 3.90–4.01 (m, 2 H, CH-4ax, CH-6ax diox), 4.04–4.13 (m, 2 H, CH-4eq, CH-6eq diox), 4.40 (d, J = 4.6 Hz, 2 H, CH2O), 4.96 (t, J = 4.6 Hz, 1 H, CH-2 diox), 7.44 (dd, 2 H, J = 7.4, 7.8 Hz, CH-3, CH-5 Ph), 7.57 (t, 1 H, J = 7.4 Hz, CH-4 Ph), 8.01 (d, 2 H, J = 7.8 Hz, CH-2, CH-6 Ph). 13_{C NMR (100 MHz, CDCl} 3): δ = 64.0 (C-5 diox), 64.9 (CH2OCO), 71.9 (C-4, C-6 diox), 98.9 (C-2 diox), 128.5 (C-3, C-5 Ph), 129.6 (C-1 Ph), 129.7 (C-2, C-6 Ph), 133.2 (C-4 Ph), 166.2 (CO). HRMS-APCI: m/z calcd for C12H15O5+ [M + H]+_{: 239.0914; found: 239.0917.} (20) trans-(5-Hydroxy-1,3-dioxan-2-yl)methyl Benzoate (2b) Yellow oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 3.02 (br s, 1 H, OH), 3.42 (dd, J = 10.4, 10.8 Hz, 2 H, CH-4ax, CH-6ax diox), 3.81–3.91 (m, 1 H, CH-5 diox), 4.22 (dd, J = 4.9, 10.4 Hz, 2 H, CH-4eq, CH-6eq diox), 4.35 (d, J = 4.6 Hz, 2 H, CH2O), 4.78 (t, J = 4.6 Hz, 1 H, CH-2 diox), 7.43 (dd, 2 H, J = 7.2, 7.8 Hz, CH-3, CH-5 Ph), 7.55 (t, 1 H, J = 7.2 Hz, CH-4 Ph), 8.04 (d, 2 H, J = 7.8 Hz, CH-2, CH-6 Ph). 13_C NMR (100 MHz, CDCl3): δ = 60.7 (C-5 diox), 64.4 (CH2OCO), 70.8 (C-4, C-6 diox), 97.8 (C-2 diox), 128.2 (C-3, C-5 Ph), 129.2 (C-1 Ph), 129.5 (C-2, C-6 Ph), 133.1 (C-4 Ph), 166.1 (CO). HRMS-APCI:

m/z calcd for C12H15O5+ [M +H]+: 239.0914; found: 239.0915. (21) trans-[5-(Tosyloxy)-1,3-dioxan-2-yl]methyl Benzoate (3b)

p-Toluenesulfonyl chloride (1.20 g, 6.3 mmol) was added at 0 °C

to a solution of 2b (1.0 g, 4.2 mmol), Et3N (8.4 mmol, 1.17 mL) in anhydrous CH2Cl2 (20 mL). The mixture was stirred at r.t. for 12 h. Ice and H2O were added, and the mixture was extracted with CH2Cl2. The organic extracts were collected and dried (Na2SO4). Crystallization from EtOAc–cyclohexane afforded the desired compound (0.86 g, 2.2 mmol, 52%). White solid; mp 83–85 °C. 1_{H NMR (400 MHz, CDCl} 3): δ = 2.46 (s, 3 H, CH3), 3.57 (dd, J = 10.4, 11.3 Hz, 2 H, CH-4ax, CH-6ax diox), 4.14 (dd, J = 5.3, 11.3 Hz, 2 H, CH-4eq, CH-6eq diox), 4.32 (d, J = 4.6 Hz, 2 H, CH2O), 4.45–4.58 (m, 1 H, CH-5 diox), 4.78 (t, J = 4.5 Hz, 1 H, CH-2 diox), 7.31–7.45 (m, 2 H, CH-2, CH-6 Ph), 7.43– 7.56 (m, 2 H, CH-3, CH-5 Ph), 7.58–7.64 (m, 1 H, CH-4 Ph), 7.81 (d, J = 8.4 Hz, 2 H, CH-3, CH-5 Ts), 8.04 (d, J = 8.4 Hz, 2 H, CH-2, CH-6 Ts). 13_{C NMR (100 MHz, CDCl} 3): δ = 21.4 (CH3), 63.9 (CH2OCO), 67.5 (C-5 diox), 67.9 (C-4, C-6 diox), 98.1 (C-2 diox), 127.6 (C-3, C-5 Ts), 128.1 (C-3, C-5 Ph), 129.5 (C-2, C-6 Ts), 129.2 (C-1 Ph), 129.9 (C-2, C-6 Ph), 133.4 (C-4 Ph), 133.7 (C-1 Ts), 145.3 (C-4 Ts), (165.8 (CO). HRMS-APCI: m/z calcd for C19H21O7S+ [M + H]+: 393.1003; found: 393.1006.

(22)

cis-{5-[5-Chloro-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl]-1,3-dioxan-2-yl}methyl Benzoate (5b)

To a suspension of 5-chlorouracil (1.2 mmol) and K2CO3 (1.2 mmol), in anhydrous DMF (10 mL), was added portionwise, under nitrogen, the tosylated compound 3b (1 mmol) and 18-crown-6 (catalytic amount). The resulting mixture was stirred and heated to reflux for 24 h. After cooling to r.t. the mixture was concentrated under vacuum. The residue was partitioned between EtOAc and H2O. The organic layer was separated, and the aqueous phase was extracted with EtOAc. The extracts were

F

C. Sorbi et al.

Letter

Syn lett

combined, washed with H2O, and dried (Na2SO4). The suspen-sion was filtered and the solvent evaporated under vacuum. The residue obtained was purified by flash chromatography to yield the desired compound (0.046 g, 0.125 mmol, 12%).

Yellow oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 4.19–4.37 (m, 4 H, CH2-4, CH2-6 diox), 4.50 (br s, 1 H, CH-5 diox), 4.53 (d, J = 3.8 Hz, 2 H, CH2O), 5.11 (t, J = 3.8 Hz, CH-2 diox), 7.53 (dd, J = 7.5, 8.0 Hz, 2 H, CH-3, CH-5 Ph), 7.69 (dd, J = 1.3, 7.5 Hz, 1 H, CH-4 Ph), 7.94 (dd, J = 1.3, 8.0 Hz, 2 H, CH-2, CH-6 Ph), 8.65 (s, 1 H, CH-6 uracil). 13_{C NMR (100 MHz, CDCl} 3): δ = 48.5 (C-5 diox), 64.0 (CH2OCO), 68.5 (C-4, C-6 diox), 99.0 (C-2 diox), 108.7 (C-5 ura-cil), 128.3 (C-3, C-5 Ph), 129.6 (C-2, C-6 Ph), 129.9 (C-1 Ph), 133.2 (C-4 Ph), 139.6 (C-6 uracil), 149.2 (C-2 uracil), 157.8 (C-4 uracil), 166.1 (CO). ESI-HRMS: m/z calcd for C16H16ClN2O6+ [M + H]+_{: 367.0691; found: 367.0692.}

(23) cis-[5-(6-Chloro-9H-purin-9-yl)-1,3-dioxan-2-yl]methyl

Ben-zoate (12b)

The compound was obtained from 3b and 6-chloropurine, fol-lowing the procedure described for 5b (0.059 g, 0.158 mmol, 15%). Dark-brown oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 4.30–4.38 (m, 2 H, CH-4ax, CH-6ax diox), 4.39–4.46 (m, 2 H, CH-4eq, CH-6eq diox), 4.49 (d, J = 4.3 Hz, 2 H, CH2O), 4.82 (br s, 1 H, CH-5 diox), 5.21 (t, J = 4.3 Hz, 1 H, CH-2 diox), 7.48 (dd, J = 7.5, 7.8 Hz, 2 H, CH-3, CH-5 Ph), 7.60 (dd, J = 1.2, 7.5 Hz, 1 H, CH-4 Ph), 8.07 (dd, J = 1.2, 7.8 Hz, 2 H, CH-2, CH-6 Ph), 8.73 (s, 1 H, CH-2 purine), 8.92 (s, 1 H, CH-8 purine). 13_{C NMR (100 MHz, CDCl} 3): δ = 48.5 (C-5 diox), 64.6 (CH2OCO), 69.0 (C-4, C-6 diox), 99.4 (C-2 diox), 128.5 (C-3, C-5 Ph), 129.3 (C-1 Ph), 129.8 (C-2, C-6 Ph), 131.0 (C-5 purine), 133.5 4 Ph), 145.4 8 purine), 151.1 4 purine), 151.5 (C-6 purine), 151.8 (C-2 purine), 1(C-6(C-6.1 (CO). ESI-HRMS: m/z calcd for C17H16ClN4O4+ [M + H]+: 375.0855; found: 375.0862. (24)

cis-[5-(2-Amino-6-chloro-9H-purin-9-yl)-1,3-dioxan-2-yl]methyl Benzoate (13b)

The compound was obtained from 3b and 6-chloro-2-amino-purine, following the procedure described for 5b (0.063 g, 0.162 mmol, 16%). Dark-brown oil. 1_{H NMR (400 MHz, CDCl} 3): δ = 4.30–4.44 (m, 4 H, CH2-4, CH2-6 diox), 4.50 (d, J = 4.2 Hz, 2 H, CH2O), 4.69 (br s, 1 H, CH-5 diox), 5.05 (br s, 2 H, NH2), 5.18 (t, J = 4.2 Hz, 1 H, CH-2 diox), 7.51 (dd, J = 7.4, 7.8 Hz, 2 H, CH-3, CH-5 Ph), 7.62 (t, J = 7.4 Hz, 1 H, CH-4 Ph), 8.09 (d, J = 7.8 Hz, 2 H, CH-2, CH-6 Ph), 8.77 (s, 1 H, CH-8 purine). 13_{C NMR (100 MHz, CDCl} 3): δ = 48.8 (C-5 diox), 64.4 (CH2OCO), 68.8 (C-4, C-6 diox), 99.5 (C-2 diox), 128.9 (C-3, C-5 Ph), 129.4 (C-1 Ph), 129.9 (C-2, C-6 Ph), 130.8 (C-5 purine), 133.5 (C-4 Ph), 139.6 (C-8 purine), 151.5 (C-6 purine), 152.7 (C-4 purine), 159.1 (C-2 purine), 166.0 (CO). ESI-HRMS:

m/z calcd for C17H17ClN5O4+ [M + H]+: 390.0964; found: 390.0969.

(25)

cis-5-Chloro-1-[2-(hydroxymethyl)-1,3-dioxan-5-yl]pyrimi-dine-2,4(1H,3H)-dione (15b)

Compound 5b (0.046 g, 0.125 mmol) was dissolved in concen-trated aq NH3 (15 mL) and stirred for 5 h in a Pyrex pressure tube. After evaporation of the solvent under vacuum, the residue was crystallized from MeOH–Et2O to give the desired

compound (0.026 g, 0.099 mmol, 79%).

White solid; mp 207–209 °C. 1_{H NMR (400 MHz, DMSO): δ =} 3.34–3.47 (m, 2 H, CH2OH), 4.01–4.12 (m, 2 H, CH-4ax, CH-6ax diox), 4.13–4.21 (m, 2 H, CH-4eq, CH-6eq diox), 4.31 (br s, 1 H, CH-5 diox), 4.71 (t, J = 4.1 Hz, 1 H, CH-2 diox), 4.95 (t, J = 6.0 Hz, 1 H, OH), 8.47 (s, 1 H, CH-6 uracil), 11.30 (br s, 1 H, NH). 13_C NMR (100 MHz, DMSO): δ = 47.5 (C-5 diox), 62.4 (CH2OH), 68.0 (C-4, C-6 diox), 100.6 (C-2 diox), 108.3 (C-5 uracil), 141.2 (C-6 uracil), 160.3 (C-2 uracil), 163.4 (C-4 uracil). Anal. Calcd for C9H11ClN2O5: C, 41.16; H, 4.22; N, 10.67. Found: C, 41.05; H, 4.14; N, 10.41. ESI-HRMS: m/z calcd for C9H12ClN2O5+ [M + H]+: 263.0429; found: 263.0428..

(26) cis-[5-(6-Amino-9H-purin-9-yl)-1,3-dioxan-2-yl]methanol

(22b)

Compound 12b (0.055 g, 0.147 mmol) was dissolved in concen-trated aq NH3 (15 mL) and placed in a reactor at 100 °C for 12 h. After solvent evaporation under vacuum, the residue was crys-tallized from MeOH–Et2O to give the desired compound (0.025 g, 0.100 mmol, 68%).

Yellow solid; mp 255–257 °C. 1_{H NMR (400 MHz, DMSO): δ =} 3.47 (dd, 2 H, J = 4.0, 6.2 Hz, CH2O), 4.02–4.16 (m, 2 H, CH-4ax, CH-6ax diox), 4.18–4.32 (m, 2 H, CH-4eq, CH-6eq diox), 4.51 (br s, 1 H, CH-5 diox), 4.69 (t, J = 4.0 Hz, 1 H, CH-2 diox), 4.90 (t, J = 6.2 Hz, 1 H, OH), 7.17 (br s, 2 H, NH2), 8.08 (s, 1 H, CH-2 purine), 8.40 (s, 1 H, CH-8 purine). 13_{C NMR (100 MHz, DMSO): δ = 49.3} (C-5 diox), 62.5 (CH2OH), 67.9 (C-4, C-6 diox), 101.3 (C-2 diox), 118.9 (C-5 purine), 139.4 (C-8 purine), 149.5 (C-4 purine), 153.0 (C-2 purine), 156.3 (C-6 purine). Anal. Calcd for C10H13N5O3: C, 47.81; H, 5.22; N, 27.87. Found: C, 47.86; H, 5.44; N, 28.03. ESI-HRMS: m/z calcd for C10H14N5O3+ [M + H]+: 252.1091; found: 252.1093.

(27)

cis-2-Amino-9-[2-(hydroxymethyl)-1,3-dioxan-5-yl]-1H-purin-6(9H)-one (23b)

Compound 13b (0.060 g, 0.154 mmol) was dissolved in MeOH and 40% NaOH aq solution was added. The reaction mixture was heated to reflux at 100 °C for 5 h, after which the solvent was removed under vacuum. The solid residue was then dis-solved in DMF, and insoluble material was removed by filtra-tion. The DMF was evaporated under vacuum to give a black oil. Crystallization from MeOH–Et2O afforded the desired com-pound (0.035 g, 0.131 mmol, 85%).

Yellow solid; mp 280–282 °C. 1_{H NMR (400 MHz, DMSO): δ =} 3.41–3.49 (m, 2 H, CH2OH), 4.01–4.13 (m, 2 H, CH-4ax, CH-6ax diox), 4.19–4.31 (m, 3 H, CH-5, CH-4eq, CH-6eq diox), 4.69 (t, J = 4.2 Hz, 1 H, CH-2 diox), 4.81–4.92 (m, 1 H, OH), 6.52 (br s, 2 H, NH2), 7.88 (s, 1 H, CH-8 purine). 13C NMR (100 MHz, DMSO): δ = 49.3 (C-5 diox), 62.5 (CH2OH), 67.9 (C-4, C-6 diox), 101.3 (C-2 diox), 116.6 (C-5 purine), 134.9 (C-8 purine), 151.3 (C-4 purine), 155.8 (C-6 purine), 158.6 (C-2 purine). Anal. Calcd for C10H13N5O4: C, 44.94; H, 4.90; N, 26.21. Found: C, 44.97; H, 5.16; N, 26.49. ESI-HRMS: m/z calcd for C10H14N5O4+ [M + H]+: 268.1040; found: 268.1041.

(28) Experimental procedures and analytical data of all compounds reported in this work can be found in the Supporting Informa-tion.